A conventional vehicle occupant detection system includes a mat-like-shaped electrostatic sensor and an occupant detection electronic control unit (ECU). In the form of a current or voltage, the electrostatic sensor outputs disturbance of a weak electric field generated between a vehicle body and a main electrode arranged inside a seat. This kind of occupant detection system is described in, for example, JP-H11-271463A.
For example, when a seat is vacant (not occupied), air is inserted between a pair of electrodes of the electrostatic sensor. When a child restraint system (CRS) is mounted to a seat, the CRS is inserted between the pair of electrodes of the electrostatic sensor. When an occupant is seated on a seat, a body of the occupant is inserted between the pair of electrodes of the electrostatic sensor. The air has a relative permittivity of approximately 1. A typical CRS has a relative permittivity of approximately 2 to approximately 5, depending on its material. A typical human body has a relative permittivity of approximately 50. As shown the above, the air, the CRS and the human body are different in relative permittivity. Thus, an electrostatic capacity between the pair of electrodes of the electrostatic sensor changes according to types of inserted object.
This change in electrostatic capacity disturbs the weak electric field between the electrodes, and is outputted as the current or voltage. Based on a value of the outputted current or voltage, the occupant detection ECU performs an occupant determination. For example, the occupant detection ECU determines whether or not the seat is vacant, determines whether or not the CRS is mounted to the seat, and determines whether or not an adult is seated on the seat. The above situation where the CRS is mounted to the seat includes a situation whether the CRS with an infant is mounted to the seat. Based on a result of this determination as to the occupant detection, an airbag ECU determines permission and prohibition of airbag deployment (inflation). Specifically, when the seat is vacant or when the CRS is mounted to the seat, the airbag is placed in an airbag deployment prohibited state. When an adult is seated on the seat, the airbag is placed in an airbag deployment permitted state.
In some cases, a heater that heats a seat by a current flowing in an electric conductor is arranged inside the seat. In this case, the heater and the electrode of the electrostatic sensor are arranged inside the seat. The heater includes the electric conductor in which the current flows. Thus, the heater influences the weak electric field between the electrodes. The occupant detection ECU may be set to perform an occupant determination by taking into account the influence of the electric conductor of the heater.
When a disconnection fault of the heater occurs, the influence of the electric conductor of the heater on the weak electric field changes. The occupant detection ECU is set to perform the occupant determination by assuming that there is no disconnection fault of the heater. When the occupant detection ECU continues to perform the occupant determination in a disconnection fault state, the occupant determination may be erroneously performed.